Air Traffic Organization chief Hank Krakowski resigned Thursday as the Federal Aviation Administration deals with a growing controversy over air traffic controllers falling asleep on the job.

FAA Administrator Randy Babbitt issued a statement that Krakowski, who had overseen the U.S. traffic system since 2007, submitted his resignation Thursday morning and that he accepted it. FAA's chief counsel, David Grizzle, will take his place until a permanent successor can be found.

"Over the last few weeks we have seen examples of unprofessional conduct on the part of a few individuals that have rightly caused the traveling public to question our ability to ensure their safety," Babbitt said in a statement. "This conduct must stop immediately. I am committed to maintaining the highest level of public confidence and that begins with strong leadership."

Five incidents have been reported recently of air traffic controllers falling asleep on the job. The latest involved a sleeping air traffic controller discovered early Wednesday in Nevada. The controller was out of communications for 16 minutes while a medical plane was landing.

No one was injured when the flight carrying at least three people landed at Reno-Tahoe International Airport. The controller has been suspended while the FAA investigates his lack of response to repeated contacts from the plane and airport staff.